A multicentre synergistic polyoxometalate-based metal-organic framework for one-step selective oxidative cleavage of β-: O -4 lignin model compounds

Article abstract of DOI:10.1039/c9gc03626a

A novel mixed-valence polyoxovanadate-based copper-organic framework, [Cu^I(bbi)]₂{[Cu^I(bbi)]₂VIV2V^V₈O₂₆}·2H₂O (NENU-MV-5, bbi = 1,1′-(1,4-butanediyl)bis(imidazole)), was facilely synthesized from routine reagents under mild hydrothermal conditions. Using NENU-MV-5 as a heterogeneous catalyst without any co-catalyst, one-step oxidative cleavage of β-O-4 lignin into phenols and aromatic acids with high catalytic activity and selectivity was realized under an oxygen atmosphere. No obvious decrease in activity was observed after five cycles, which indicates the excellent stability and sustainability of NENU-MV-5. The perfect catalytic performance of NENU-MV-5 can be attributed to the multi-site synergistic effect of the mixed-valence V^V-O-V^IV sites on polyoxovanadate for the oxidation of β-O-4 alcohol to β-O-4 ketone and the Cu(i) sites on the framework for the rapid cleavage of the C_α-C_β bond of β-O-4 ketone. This system represented the first co-catalyst-free example for the one-step selective degradation of lignin catalyzed by a well-defined crystalline catalyst with definite composition and structure in a single solvent.

Full text of DOI:10.1039/c9gc03626a

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A multicentre synergistic polyoxometalate-based
metal–organic framework for one-step selective
oxidative cleavage of β-O-4 lignin model
compounds†
Cite this: Green Chem., 2020, 22,
48
2
Hong-Rui Tian,‡ Yi-Wei Liu,‡ Zhong Zhang, Shu-Mei Liu, Tian-Yi Dang, Xiao-Hui Li,
Xiu-Wei Sun, Ying Lu
and Shu-Xia Liu
*
I
I
A
novel mixed-valence polyoxovanadate-based copper–organic framework, [Cu (bbi)]
2
{[Cu (bbi)]
2
IV
2
V
8 2
V V O26}·2H O (NENU-MV-5, bbi = 1,1’-(1,4-butanediyl)bis(imidazole)), was facilely synthesized from
routine reagents under mild hydrothermal conditions. Using NENU-MV-5 as a heterogeneous catalyst
without any co-catalyst, one-step oxidative cleavage of β-O-4 lignin into phenols and aromatic acids with
high catalytic activity and selectivity was realized under an oxygen atmosphere. No obvious decrease in
activity was observed after five cycles, which indicates the excellent stability and sustainability of NENU-MV-5.
The perfect catalytic performance of NENU-MV-5 can be attributed to the multi-site synergistic effect of the
V
IV
Received 22nd October 2019,
Accepted 28th November 2019
mixed-valence V –O–V sites on polyoxovanadate for the oxidation of β-O-4 alcohol to β-O-4 ketone and
the Cu(I) sites on the framework for the rapid cleavage of the C –C bond of β-O-4 ketone. This system rep-
α
β
DOI: 10.1039/c9gc03626a
resented the first co-catalyst-free example for the one-step selective degradation of lignin catalyzed by a
rsc.li/greenchem
well-defined crystalline catalyst with definite composition and structure in a single solvent.
1
6–18
vanadium, cobalt, etc.) complex catalysts,
which can
Introduction
realize the one-step conversion of lignin into small molecular
1
–3
14,19,20
Lignin, as a renewable biopolymer, is abundant in plants.
aromatics; (ii) organocatalysts,
in this system, nitroxyl-
Relatively stable C–O and C–C bonds link the primary units of based co-catalysts (such as TEMPO, etc.; TEMPO = 2,2,6,6-tetra-
lignin, in which the β-O-4 bonds account for ca. 50% of all methylpiperidine-N-oxyl) are indispensable to complete the
4
–6
linkages. Therefore, in order to obtain high value added aro- conversion of β-O-4 alcohol into β-O-4 ketone; and (iii) in-
1
5,21,22
matics, depolymerisation of lignin through the efficient cata- organic salt (VOSO , CuCl, etc.) catalysts.
A representa-
4
lytic cleavage of the β-O-4 bonds has attracted much attention tive example is VOSO
4
/TEMPO, which can catalyze the oxi-
7
from both industrial and academic researchers. The reported dation of β-O-4 alcohol to β-O-4 ketone in acetonitrile, followed
methods for the targeted cleavage of β-O-4 bonds in lignin by the rapid C –C bond cleavage of β-O-4 ketone catalysed by
α
β
8
9,10
11,12
21
include acid–base hydrolysis, pyrolysis,
their combinations,
oxidation,
2
and Cu(OAc) /1,10-phenanthroline in methanol. Although these
among which the oxidation approach homogeneous catalytic systems are effective in catalysing the
1
3,14
is considered as the most effective strategy owing to the well- oxidative cleavage of β-O-4 lignin, they suffer from the
retained premier functional groups (methoxyl, etc.) of the difficulty in separation and recovery. Heterogeneous catalysis
1
5
lignin polymer.
is an alternative strategy to resolve these problems. Recently, a
Homogeneous catalytic systems are widely used for the Pd/CeO catalyst has been reported for the one-step oxidative
2
2
selective oxidative cleavage of lignin, and these catalysts can be cleavage of 2-phenoxy-1-phenylethanol using O as an oxidant
2
3
divided into three categories: (i) transition metal (such as in MeOH. Co-based composite catalysts also exhibited high
performance for the oxidative cleavage of β-O-4 lignin with
2
4,25
solid bases (K
2
CO
3
, NaOH, etc.) as co-catalysts.
However,
Key Laboratory of Polyoxometalate Science of the Ministry of Education, College of
Chemistry, Northeast Normal University, Changchun, Jilin 130024, P. R. China.
E-mail: liusx@nenu.edu.cn
their ill-defined composition and structure prevented the
further study of their catalytic mechanisms. More importantly,
the requirement of two catalysts, two solvents, and usually the
assistance of co-catalysts (such as nitroxyl species, acids,
bases, etc.) in certain catalytic systems severely limit their prac-
tical application. Therefore, the development of efficient and
†
Electronic supplementary information (ESI) available. CCDC 1912680. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9gc03626a
‡
These authors contributed equally to this work.
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Scheme 1 One-step oxidative cleavage of the β-O-4 bond by synergis-
tic catalysis.
sustainable heterogeneous catalysts with well-defined compo-
sition and structure but requiring no co-catalyst is highly
desirable.
In this work, we synthesized a novel polyoxometalate-based
2
6,27
I
I
metal–organic framework (POMOF),
[Cu (bbi)]
2 2
{[Cu (bbi)]
IV
V
V V O26} (NENU-MV-5; bbi = 1,1′-(1,4-butanediyl)bis(imid-
2
8
azole)). Owing to the synergistic catalytic effect of multiple
active sites in NENU-MV-5, it can catalyze the one-step oxi-
dative cleavage of β-O-4 lignin using oxygen as an oxidant
without any co-catalyst. In particular, the V –O–V sites with
mixed-valence (MV) vanadium act as the active centre for the
oxidation of β-O-4 alcohol to β-O-4 ketone; then the obtained
β-O-4 ketone is rapidly converted into target phenols and aro-
4
−
Fig. 1 Coordination mode of Cu2 (a), Cu1 (b) and [V10
O
26
]
(c). (d) 2D
2
−
layer of {[Cu(bbi)]
cules are omitted for clarity.
2 10 26
V O } . Hydrogen atoms and lattice water mole-
V
IV
5
oxidation states of V ions (V3 and V6) in VO are approximately
matic acids on Cu(I) sites through C
α β
–C bond cleavage
close to +4 (Table S2†). X-ray photoelectron spectroscopy (XPS)
was carried out to further confirm the oxidation states of Cu
ions and V ions in NENU-MV-5. The Cu 2p3/2 peak at 932.2 eV
in XPS demonstrates that the oxidation state of Cu ions in
(Scheme 1). To the best of our knowledge, NENU-MV-5 is the
first crystalline heterogeneous catalyst with well-defined com-
position and structure for the degradation of lignin.
2
9
NENU-MV-5 is +1 (Fig. S3a†). Two legible peaks with electron
binding energies of 517.15 eV and 516.2 eV can be assigned to
Results and discussion
30
V(V) 2p3/2 and V(IV) 2p3/2
,
respectively, and their peak area
Composition and structural analysis of NENU-MV-5
ratio is about 4 : 1 (Fig. S3b†). These results are consistent with
the BVS calculation results and balance the skeleton charges
of NENU-MV-5. It should be noted that reduced coordinatively
unsaturated metal sites (CUSs) have been widely accepted that
Blue-violet strip NENU-MV-5 crystals were synthesized from
sodium metavanadate, hydrazine hydrate, bbi and copper
nitrate trihydrate under mild hydrothermal conditions. The
I
I
can adsorb O
2
and activate O
2
to produce active oxygen species
, result-
Remarkably,
molecular formula was determined as [Cu (bbi)] {[Cu (bbi)]
2
2
through interacting with the lone pair electrons of O
ing in charge transfer from CUSs to O2.
2
IV
V
V V O26}·2H
2
O by single-crystal X-ray diffraction, elemental
2
8
31,32
analyses and thermogravimetric analysis (TGA) (Fig. S1b†).
NENU-MV-5 exhibited a 3D supramolecular structure (Fig. S4†)
NENU-MV-5 combines two kinds of reduced CUSs (V(IV) and
Cu(I)) in one compound, which is expected to be of great
potential for O activation.
2
+
constructed by a 1D cationic chain [Cu(bbi)] (Fig. 1a) and a
2
−
2
D anionic layer {[Cu(bbi)]
2
V
10
O
26
}
(Fig. 1d) through electro-
static interaction. In this structure, Cu ions have two kinds of
coordination modes. As shown in Fig. 1, Cu2 coordinates with
two nitrogen atoms from two bbi molecules in a linear geome-
Catalytic performance of NENU-MV-5 for lignin model
compounds
try to form the 1D chain (Fig. 1a) and Cu1 presents a four-co- 1-Phenyl-2-phenoxyethanol (1) is a widely used model com-
ordinated structure with two nitrogen atoms from two bbi pound for the study of lignin catalytic degradation. However,
molecules and two terminal oxygen atoms from two {V10} clus- the oxidative cleavage of 1 usually involves two steps with two
1
4,33
24,25
ters to construct the 2D layer (Fig. 1b and d). Bond valence catalysts,
the addition of a co-catalyst,
and sometimes
2
8
19,21
sum (BVS) calculations demonstrated the +1 oxidation state even two solvents.
We found that NENU-MV-5 could cata-
4
−
of all Cu ions (Table S2†). [V10
O
26
]
exhibited a flat spherical lyze the oxidative cleavage of 1 to aromatics in one step
structure with eight VO tetrahedra and two VO tetragonal pyra- without any co-catalyst using only methanol (MeOH) as the
4
5
mids linked by the vertex-sharing of O atoms (Fig. 1c and d). sole solvent. The catalytic reaction was carried out at 100 °C
The BVS calculations revealed that the oxidation states of V under an O atmosphere (0.4 MPa). For a better dispersion in
ions (2V1, 2V2, 2V4, and 2V5) in VO are close to +5, and the reaction media, NENU-MV-5 crystals were ground and the
2
4
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Brunauer–Emmett–Teller (BET) surface area of the used cata- Table 1 Catalytic oxidative cleavage of
1
and 1a with different
a
2
−1
catalysts
lyst was 15 m
g
(Fig. S5†). We monitored the reaction
process by gas chromatography with flame ionization (GC-FID)
and the products were identified by gas chromatography with
mass spectrometry (GC-MS); more than 99% of compound 1
was converted within 12 hours, and the yields of phenol (1b),
benzoic acid (1c) and methyl benzoate (1d) were 60%, 9.5%
and 90%, respectively. Trace 2-phenoxy-1-phenylethanone (1a)
was also detected with a yield of less than 1% (Table S3,† entry
d
Yield (%)
Entry Catalyst
Conv. (%) 1a
1b
1c
1d
1
). It is noteworthy that the major products of the oxidative 1 as the substrate
2
1,34
1
2
3
4
5
6
7
VOSO
NaVO
CuCl
4
44.6
<1
<1
44.6 n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
90.0
cleavage of β-O-4 lignin are usually 1b and/or 1c,
whereas
3
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
1
d (90%) was detected as the main product in our reaction
system. Considering the presence of Lewis acid sites (Cu/V) on
NENU-MV-5, we speculated that 1d was generated from the
esterification reaction of the resultant 1c and MeOH solvent.
In order to prove our conjecture, we replaced substrate 1 with 1a as the substrate
commercially available benzoic acid (1c) under both N and O
Cu(NO3)2
(Et N)
VOSO
NENU-MV-5
<1
4
4
(V10
+ NaVO
O
26)·H
3
2
O
96.7
30.0
>99
96.7 n.d.
30.0 n.d.
b
4
<1
60.0 9.5
c
8
9
CuCl
Cu(NO )
3 2
85.0
65.0
—
—
55.0 32.0 51.0
51.5 15.4 71.0
2
2
c
atmospheres keeping the other conditions the same, and a
large amount of methyl benzoate (1d) was indeed detected n.d. = not detected. Reaction conditions: Substrate (0.5 mmol), cata-
a
lyst (0.1 mmol, based on V or Cu), MeOH (2 mL), O (0.4 MPa), 100 °C,
2
(Table S4,† entries 1 and 2).
b
c
12 hours. Catalyst (0.02 mmol VOSO
Yield was determined by GC using biphenyl as an internal standard.
4
+ 0.08 mmol NaVO
3
). 2 hours.
The previously reported catalytic process for the selective
d
oxidative cleavage of β-O-4 lignin usually requires two catalysts,
specifically, first oxidizing lignin β-O-4 alcohol (1) into β-O-4
ketone (1a) with one catalyst, followed by the C
α β
–C bond clea-
oxidation states of V in these catalysts, we can deduce that
V(IV) is the catalytically active site for the oxidation of 1 to 1a,
but it is not active for the C–C bond cleavage of 1a to 1b and
vage of 1a into phenol (1b) and benzoic acid (1c) with
1
9,21
another.
In contrast, in our work, NENU-MV-5 was used as
the only catalyst and the obtained main products were 1b, 1c
and 1d with only a trace amount of 1a being detected. This
result indicated a cascade reaction process that 1a was
degraded immediately after being generated from the oxi-
dation of 1. To clarify the tandem catalytic process, we
replaced 1 with stoichiometric intermediate 1a without chan-
ging the other reaction condition, and 1a was completely con-
verted into 1b, 1c and 1d in less than 2 h (Table S3,† entry 3)
whereas the conversion of 1 was only 30% (Table S3,† entry 4)
at the same time. The obvious differences demonstrated that
the oxidation of 1 to 1a was a rate-determining step for the
degradation of 1 on NENU-MV-5. According to the related
1
c. In contrast, copper(I/II) was inactive for the oxidation of
β-O-4 alcohol to β-O-4 ketone; it was catalytically active for the
cleavage of 1a to 1b and 1c (Table 1, entries 8 and 9). It was
also observed that the catalytic activity of (Et N) (V O )·H O
involving MV-vanadium was distinctly higher than that of
VOSO , which might be caused by the collaborative effect of
4
4
10 26
2
4
V
IV
MV-vanadium in the {V10} cluster. The V –O–V mode could
promote electron transfer through intervalence charge transfer
4
0–42
(
IVCT) to expedite the catalytic reaction.
For comparison, a
physical mixture of VOSO and NaVO was used to catalyze the
oxidative cleavage of 1; the conversion (merely 30%) was
obviously lower than that of (Et N) 26)·H O (Table 1,
entries 5 and 6), which could be ascribed to the separated V(IV)
and V(V) lacking IVCT between them. Overall, the role of each
active centre of NENU-MV-5 in the oxidative cleavage of 1
could be clarified; V –O–V was effective for the catalytic oxi-
dation of 1 to 1a and then Cu(I) catalysed the C
vage of 1a to 1b and 1c.
4
3
(V10
O
2
3
5–38
4
4
literature,
we proposed a synergistic catalysis of multiple
active sites to explain the one-step oxidative cleavage of 1 on
V
IV
NENU-MV-5. Firstly, MV-vanadium in V –O–V of NENU-MV-5
catalysed the oxidation of the C –OH bond of 1 to 1a; sub-
sequently, the C –C bond of 1a was promptly cleaved to
α
V
IV
α
β
α β
–C bond clea-
obtain 1b and 1c on Cu(I) sites.
To confirm the aforementioned assumptions, a range of
control experiments were carried out using VOSO
4 3
, NaVO ,
Reaction mechanism of NENU-MV-5 for catalysing the
conversion of β-O-4 lignin model compounds
CuCl, Cu(NO ) and (Et N) (V O )·H O containing V(IV), V(V),
3
2
4
4
10 26
2
Cu(I), Cu(II) and MV-{V10} cluster as catalysts, respectively
Table 1). (Et N) (V10 26)·H
according to the literature in which [V O ] had the same lignin was proposed based on the experimental results. The
(
4
4
O
2
O (Fig. S6†) was synthesized The mechanism of NENU-MV-5 for the catalytic cleavage of
3
9
4−
1
0 26
structure as that in NENU-MV-5. These catalytic results showed catalytic oxidative cleavage of lignin model compound 1 with
V
IV
that VOSO
catalysts gave only 1a in 44.6% and 96.7% yields, respectively the C –OH bond oxidation of 1 to 1a, and then, the C –C
β
4 4 4 2
and (Et N) (V10O26)·H O, both possessing V(IV), as NENU-MV-5 involves two processes: firstly, V –O–V catalyzes
α
α
(
Table 1, entries 1 and 5). In contrast, V(V)-containing NaVO
and V-free CuCl and Cu(NO as catalysts exhibited no cata- necessary owing to the suppressive activities in the absence of
lytic activity (Table 1, entries 2–4). Considering the different O (Table S3,† entries 5 and 6). A radical reaction mechanism
3
bond of 1a is cleaved on Cu(I). In both the processes, oxygen is
3 2
)
2
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for the oxidative cleavage of lignin with oxygen as an oxidant the five-coordinated V(IV) geometry in the MV-{V10} cluster is
has been reported, in which the main active species might be similar to the vanadium centre configuration of the formation
•
−
•
•
43
2−
53
O
2
, HOO and/or OH. MeOH is an effective scavenger of of [O]
vacancy in polyoxovanadate. Oxygen vacancy is
•
44
•
− 54
2 2
OH, and it is used as a solvent in the oxidative cleavage of 1; inclined to absorb O molecules and convert them into O .
•
•
−
thus OH would not be the active species. To confirm the These results illustrate that O could be generated from the
2
2
active oxygen species involved in the oxidative cleavage of 1, an interaction of V(IV) with O . The tetracoordinated V(V) in
electric paramagnetic resonance (EPR) test was performed on NENU-MV-5 can interact with 1-phenyl-2-phenoxyethanol (1) to
NENU-MV-5 with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), form a five-membered ring to activate the C –H bond to
α
which has been regarded as the most commonly used trapper promote the rapid oxidation of 1 to 1a, and this phenomenon
4
5
34
for detecting spin-reactive species. The characteristic peak of has been confirmed by Hanson and co-workers. Accordingly,
•
−
V
IV
the DMPO– O adduct in both MeOH (Fig. 2a) and aceto- we proposed that the high catalytic performance of V –O–V
2
nitrile (Fig. S7†) was observed as compared to the absence of catalyses the oxidation of 1 to 1a, which could be attributed to
4
6,47
NENU-MV-5,
indicating that NENU-MV-5 could produce the synergistic effect of pentacoordinated V(IV) for activating
•
−
•
−
O2 by interacting with dissolved oxygen in the solvent. O2 to O and tetracoordinated V(V) for activating the C –H
2
α
•
−
+
Notably, the oxidative cleavage of 1 does not occur in aceto- bond of 1. The obtained O
2
combines with H from MeOH to
may not be the active give HOO , which abstracts the hydrogen atom of C to achieve
species of the oxidative cleavage of 1. According to the previous the oxidation of 1 to 1a.
•
−
•
nitrile (Table S3,† entry 2); thus O
2
α
4
8 •
−
reports,
O
2
alone is not effective in directly reacting with
Next, we investigated the reaction of Cu(I) sites in
•
β-O-4 alcohol, while HOO is able to abstract the C
α
–H of β-O-4 NENU-MV-5 catalysed C –C bond cleavage of 1a to 1b and 1c
α
β
alcohol to β-O-4 ketone. Therefore, it is reasonable to speculate using O2 as an oxidant. According to our previous experi-
•
that HOO is the active species in which 1 is oxidized to 1a, as mental results, the obtained methyl benzoate (1d) in the oxi-
•
•
−
+
HOO is acquired by the combination of O
MeOH, which also explains the phenomenon that the oxidative resultant benzoic acid (1c) and solvent MeOH (Table S4,†
cleavage of 1 could proceed in MeOH rather than in aceto- entries 1 and 2); so we deduce that the C –C bond of 1a is
nitrile. In order to understand the activation of O by firstly cleaved. To confirm our conjecture, the oxidative clea-
NENU-MV-5 to produce O2 , the O2 adsorption/desorption vage of methyl benzoylformate (the intermediate of the prefer-
2
with H from dative cleavage of 1a is generated by the esterification of the
α
β
2
•
−
2
3,24
test of NENU-MV-5 was performed. As shown in Fig. 2b, an ential cleavage of the C
obvious hysteresis in the O desorption isotherm indicated the NENU-MV-5 was carried out under the same conditions as
presence of a strong interaction between NENU-MV-5 and those for 1a and no 1c and 1d were detected (Scheme S2†),
β
–O bond of 1a, Scheme S1†)
by
2
4
9
oxygen. Generally, reduced CUSs could adsorb and activate suggesting that the C
β
–O bond was not first broken but the
O
2
. However, there are two kinds of reduced CUSs in –C bond was first broken in the oxidative cleavage of 1a. It
C
α
β
NENU-MV-5, V(IV) and Cu(I). It is well known that coordina- is well known that coordinatively unsaturated Cu(I) can react
•
tively unsaturated Cu(I) can interact with oxygen to form a with oxygen to produce Cu(II)–OO , which is an effective
5
5
superoxocopper(II) or μ-peroxocopper(II) complex but not free oxidant for the cleavage of the C–C bond. Hence, in the
•
− 50–52
O2
,
and our previous experimental results show that process of the oxidative cleavage of 1a catalyzed by Cu(I), we
to give Cu(II)–
bond cleavage of 1a to 1c and
obtained from the O activated by V(IV). In addition, the BVS phenol formate, and then the decarboxylation of the obtained
Cu(I/II) is not catalytically active for the oxidation of 1 to 1a proposed that Cu(I) firstly interacted with O
(
2
•
−
•
Table 1, entries 3 and 4). Hence, we deduce that
O
2
α β
is OO , which catalysed the C –C
2
calculations demonstrate that the V(IV) atoms are positioned phenol formate to phenol (1b) and CO
on the top and bottom of the MV-{V10} cluster (Fig. 1c), and water test confirmed the production of CO
Fig. S8†). Remarkably, the amount of CO (1.65 mmol) calcu-
2
occurred. The lime-
2
in the reaction
(
2
3
lated from the obtained CaCO (165 mg, 1.65 mmol) collected
with limewater (Scheme S3†) was obviously higher than the
amount of CO2 (0.5 mmol) produced by the oxidative de-
carboxylation of 1 (Table S5†). Considering that phenol is
•
56
2 2
easily oxidized by HOO to CO and H O (Scheme S4†), the
generation of extra CO (1.15 mmol equivalent to 0.19 mmol
2
phenol) which was comparable to the reduced phenol
(
0.2 mmol) indicated that part of the phenol produced by the
oxidative cleavage of 1 was oxidized to CO and H O.
2
2
Based on the experimental results and the above discus-
sion, possible mechanism was proposed for the
catalytic reaction process (Scheme 2). Initially, MV-vanadium
a
V
IV
•
−
in V –O–V of NENU-MV-5 oxidizes β-O-4 alcohol to β-O-4
ketone via synergistic catalytic action. In particular, pentacoor-
Fig. 2 (a) DMPO spin trapping EPR spectra for DMPO– O
species in methanol. (b) adsorption/desorption isotherms of
NENE-MV-5, P = 0.4 MPa.
2
radical
O
2
V
IV
0
dinated V(IV) and tetracoordinated V(V) of V –O–V (a) interact
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Finally, j rearranges to k, which experiences the cleavage of the
Cα–Cβ bond and decarboxylation to produce benzoic acid,
phenol and CO . Methyl benzoate is obtained by the esterifica-
2
tion reaction of MeOH and benzoic acid under the catalysis of
Lewis acids (Cu/V in NENU-MV-5).
Oxidative cleavage of other lignin model compounds with
NENU-MV-5
It is known that β-O-4 lignin model compounds with more
functional groups, such as the methoxyl group, C –OH, etc.,
γ
1
3,23,57
are structurally more similar to native lignin.
Hence, we
investigated the catalytic behaviours of NENU-MV-5 towards
the oxidative cleavage of a series of β-O-4 lignin model com-
pounds with methoxyl and C –OH groups. As shown in
γ
Scheme 2 Proposed catalytic mechanism of NENU-MV-5.
Table 2, NENU-MV-5 could catalyze the oxidative cleavage of
lignin model compounds with different amounts of methoxyl
with oxygen and β-O-4 alcohol, respectively (b). V(IV) adsorbs groups to afford the corresponding value-added aromatics
•
−
and activates oxygen to afford O
2
(c), and V(V) interacts with under the same conditions as 1 (Table 2, entries 2–7).
β-O-4 alcohol to form a stable five-membered ring to activate Although the C –OH group makes the oxidative cleavage of the
γ
•
−
+
•
21
C –H (e). O (c) combines with H from MeOH to give HOO
α β
C –C bond more difficult, NENU-MV-5 can still catalyse the
α
2
(
α
γ
d), which extracts hydrogen from C –H (e) and further forms oxidative cleavage of the lignin model compounds with C –OH
the transition state f. Subsequently, the transition state f to phenols, aromatic acids and esters in a good yield by appro-
undergoes a single electron transfer to give intermediate β-O-4 priately increasing the reaction temperature (120 °C) and
ketone (g). Noticeably, the process of the oxidation of β-O-4 prolonging the reaction time (18 hours) (Table 2, entry 8).
2
3
alcohol to β-O-4 ketone activates
β
C –H.
Cu(I) (h) in These results demonstrated that NENU-MV-5 can efficiently
•
NENU-MV-5 reacts with oxygen to give Cu(II)–OO (i), where and selectively catalyze the β-O-4 lignin model compounds
Cu(II) coordinates with the carbonyl oxygen of β-O-4 ketone with characteristic functional groups (methoxyl and C –OH
γ
•
and C
β
–H is oxidized by Cu(II)–OO to the transition state j. groups, which are usually present in native lignin) to give aro-
a
Table 2 Selective oxidative cleavage of lignin model compounds with NENU-MV-5
Product
Product
Phenol
Phenol
Acid
6.5
Ester
90.0
Acid
4.4
Ester
94.8
c
c
Entry
1
Model compounds
Yield (%)
Entry
5
Model compounds
Yield (%)
60.0
55.0
62.0
58.0
65.0
53.0
54.5
42.0
2
3
4
9.0
7.8
8.3
91.0
91.5
91.1
6
7
7.6
5.3
4.5
92.0
94.0
92.8
b
8
a
b
c
Reaction conditions: Substrate (0.5 mmol), NENU-MV-5 (0.01 mmol), MeOH (2 mL), O
was determined by GC using biphenyl as an internal standard.
2
(0.4 MPa), 100 °C, 12 hours. 120 °C, 18 hours. Yield
252 | Green Chem., 2020, 22, 248–255
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Paper
matics, and indicated its potential to catalyse the oxidative
cleavage of native lignin.
Compared with the reported catalysts for the oxidative clea-
vage of β-O-4 lignin (Table S6†), NENU-MV-5 exhibited several
advantages. First of all, as
NENU-MV-5 showed comparable catalytic activity with homo-
geneous catalysts (Table S6,† entries 2 and 3).
the one-step degradation of lignin model compounds was rea-
lized in NENU-MV-5 by the synergy of multiple sites, whereas
two steps with two catalysts were required in certain homo-
geneous catalytic systems (Table S6,† entries 9–11).
Thirdly, NENU-MV-5 exhibited higher activity than palladium-
a heterogeneous catalyst,
Recycling experiments
3
4,58
Secondly,
NENU-MV-5 exhibited excellent stability and recyclability.
There was no significant decrease in activity after five cycles
(the conversion of 1 was maintained at 99–96%). Moreover, the
yields of phenol, benzoic acid and methyl benzoate almost
remained constant during the five cycles (Fig. 3). Powder X-ray
diffraction (PXRD) and Fourier-transform infrared (FTIR)
spectra (Fig. 4a and b) of the recovered catalyst matched well
with those of a fresh catalyst, indicating its structural integrity.
In addition, there was no noticeable difference in XPS results,
which demonstrates the well-maintained oxidation states of
cuprous ions and vanadium ions in the catalyst before and
after the reaction (Fig. 4c and d).
1
4,19,20
and cobalt-based heterogeneous catalysts (Table S6,†
2
3–25
entries 5–7),
and does not require the assistance of any co-
CO , NaOH, etc.). Hence, NENU-MV-5 can
catalysts (such as K
2
3
be considered as a sustainable and environmentally friendly
catalyst for the oxidative cleavage of lignin into value-added
aromatics.
Conclusions
In conclusion, we synthesized a MV-{V10} cluster-based Cu–
MOF, NENU-MV-5, containing multiple catalytically active
sites. Using NENU-MV-5 as a heterogeneous catalyst without
any co-catalyst, β-O-4 lignin can be degraded into high value
added aromatics in one step with high catalytic activity and
recyclability. The catalytic mechanism was proposed and con-
firmed based on the experimental results. This work demon-
strates that multiple active sites can be integrated into one
material through constructing a polyoxometalate-based metal–
organic framework, which provides an insight into designing
multifunctional catalysts for organic transformations.
Fig. 3 The recyclability of NENU-MV-5. Reaction conditions: Lignin
model compound 1 (0.5 mmol), NENU-MV-5 (0.01 mmol), MeOH
(
2
2 mL), O (0.4 MPa), 100 °C, 12 hours, biphenyl as an internal standard.
Experimental
Materials and instrumentation
All of the reagents and solvents were of analytical grade, pur-
chased from commercial sources and used without further
purification. The ligand 1,1′-(1,4-butanediyl)bis(imidazole)
(
bbi) and lignin model compounds were synthesized according
5
9,60
1
to the literature methods
and characterized by H NMR
1
(
the synthesis and H NMR spectra are shown in the ESI†).
Elemental analyses (C, H and N) were performed on a
PerkinElmer 2400 CHN elemental analyzer. V and Cu were
analyzed using
a PLASMA-SPEC(I) ICP atomic emission
spectrometer. FTIR spectra were collected (as KBr-pressed
pellets) using a Nicolet 170SXFT-IR spectrophotometer in the
range of 400–4000 cm− . Thermogravimetric analysis (TGA)
was conducted under a nitrogen atmosphere with a heating
rate of 10 °C min− on an SDT 2960 Simultaneous DSC-TGA
from TA Instruments up to 600 °C. Powder X-ray diffraction
1
1
(
PXRD) measurements were performed on a SmartLab instru-
ment with Cu Kα (λ = 1.5418 Å) radiation and X-ray 40 kV/
3
1
0 mA over the angular range 2θ 5°–50° at a scan rate of
Fig. 4 The FTIR spectra (a), PXRD patterns (b) and XPS results (oxidative
states of cuprous ions (c) and vanadium ions (d)) of the recovered cata-
lyst NENU-MV-5 after five catalytic reactions.
0° min− . Electron paramagnetic resonances (EPR) were
1
obtained using an MS5000 EPR spectrometer. Nitrogen and
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Green Chemistry
oxygen adsorption/desorption measurements were performed MeOH were added to a 10 mL autoclave reactor with an
by using a Hiden isochema IGA 100B instrument at 77 K and internal Teflon insert. Then, the reactor was charged with 0.4
2
2
98 K, respectively. X-ray photoelectron spectra (XPS) were col- MPa O , heated to 100 °C and stirred for 12 h. The reactor was
lected using a Thermo ESCALAB 250 X-ray photoelectron cooled to room temperature, the catalyst was centrifuged and
1
spectrometer. H NMR spectra were recorded using a Bruker the supernatant was diluted with MeOH. The conversion of
1
Avance 600 ( H: 600 MHz) system at ambient temperature. lignin model compounds and the products were confirmed by
Data are reported as chemical shifts in ppm relative to TMS gas chromatography-mass spectrometry (GC-MS).
1
(
0.00 ppm) for H. The following abbreviations are used to
explain the multiplicities: s = singlet, br s = broad singlet, d = Recycling experiment of NENU-MV-5
doublet, t = triplet, q = quartet, and m = multiplet. The conver-
The reusability of the catalyst NENU-MV-5 was tested using
-phenoxy-1-phenylethanol (1) as the substrate. After the com-
pletion of the first catalytic reaction, the catalyst was recovered
by centrifugation and then washed three times with MeOH.
The recovered NENU-MV-5 was dried under air overnight and
reused for the next run under the same conditions.
sion of lignin model compounds and the products were con-
firmed by gas chromatography (GC) using an Agilent 7800A
instrument equipped with an HP-5 column. The products were
identified by gas chromatography with mass spectrometry
2
(GC-MS) on an Agilent 7890A-5975C instrument. The products
were quantified by GC using a Shimadzu GC-2014 instrument
equipped with an SH-Rtx-Wax column.
Conflicts of interest
Synthesis of NENU-MV-5
NaVO3 (122 mg, 1 mmol) and N H ·H O (80%, 19 μL,
2
4
2
The authors declare no competing financial interest.
0
2
.32 mmol) were added into 10 mL of H O. The resulting
mixture was stirred for 1 hour at room temperature, and then
bbi (95 mg, 0.5 mmol) and CuNO ·3H O (121 mg, 0.5 mmol)
were consecutively added. Then the mixture was stirred for
3
2
Acknowledgements
another 1 hour at room temperature. The pH of the mixture The authors thank the National Natural Science Foundation of
was about 5.0. Then the mixture was placed in a 23 mL Teflon China (Grant No. 21872021, 21671033 and 21901135).
reactor and kept under autogenous pressure at 150 °C for 3
days. After this the mixture was cooled to 140 °C in 1 hour and
kept at this temperature for 3 hours. Then the mixture was
cooled to room temperature at a rate of 10 °C per hour, and
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